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CHAPTER IV

GEOCHEMISTRY

In this chapter, major element geochemistry of selected rock types are

presented and discussed. In all 14 representative rock samples were chemically

analysed including met-pelites, gneisses, mafic and basic granulites from both Sargur

Schist Belt and Granulites (Biligirirangan and Male Mahadeswara Granulites). The

analyses of rocks were plotted on different compositional and variation diagrams to

understand their geochemical characters.

Representative samples of the different lithologies were chosen and analyses

were carried out by XRF methods in Centre for Earth Science Studies (CESS),

Trivandrum. The main aim of this chapter is to arrive at the nature of the protoliths of

acid and basic granulites which form the bulk of the rock types of the area including

granulite bodies and pelites.

An attempt has been made to compare the geochemical features of Sargur

Schist Belt with Granulites (Biligirirangan and Male Mahadeswara Granulites). Major

elemental chemistry of rocks has been used for classification of acid and basic

granulites to understand geochemical variation using Harker’s variation diagram and

ternary plots.

The composition and source of various lithological units in granulite terrain is

difficult to understand as the terrain has suffered intense degree deformation and high

pressure and temperature metamorphism and migmatization. As a result, the original

characteristics of the rocks have been partly to completely obliterated. Though the

mineral assemblages of the granulites give us an idea about the metamorphism, it is

difficult to understand the protolith characteristics for the granulites. However, a

careful sampling of different rock types in the granulite terrane avoiding migmatites

major and trace element analyses of relatively undeformed rocks can be used in

evaluating the probable source rocks for the granulites. This is because geochemical

data of granulites serve finger prints of the composition and evolution of granulites,

which have been formed at deeper level of continental crust and also give us an

insight into the crust-mantle interaction. Therefore, knowledge of geochemistry of

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granulite is essential for understanding the protoliths characters, tectonic setting

including many chemical changes during retrogression within the shear zone as well

as elemental mobility, if any during high-grade metamorphism and retrograde

alteration.

Geochemical study of the widespread charnockitic-enderbitic granulites

exposed in south India has shown that they either exhibit igneous or sedimentary

protolith characteristics (Rudnick, 1985). The Archaean Biligirirangan granulites are

dominantly Tonalitic to trondhjemitic granodiorite (TTG) in composition and are

comparable to the cratonal gneisses in the Gorur-Hassan area (Bhaskar Rao et al.,

1991; Basavarajappa1992; Raith et al., 1999) in contrast to the late Archaean Nilgiri

granulites, which have been classified as belonging to greywacke type of sediments,

metamorphosed around 2,500 m.y (Raith et al., 1999).

Geochemical analyses for only 14 samples from SSB and BRG/MMG terrane

have been obtained for the present study. With the available data an attempt has been

made to identify the protoliths characters and elemental mobility during granulite

facies metamorphism and during retrogression of granulites in the area investigated.

4.1 SARGUR SCHIST BELT (SSB)

4.1.1 Metapelites

Metapelites samples analysed include garnet-kyanite-biotite ±sillimanite

bearing assemblages. The analyses are given in Table.4.1.

The metapelites contains SiO2 of 64.53wt% while Al2O3 16.19wt%. Fe2O3

value of 8.11wt% is reflected by the abundant garnet and spinel in the metapelites.

The SiO2/Al2O3 and K2O/Na2O ratios are considered as indicators of evolutionary

changes in the compositions continental crust (Viezer, 1973; Schwab, 1978) and

provenance (Condie and Wronkewicz, 1990). SiO2/Al2O3 ratio of this metapelites is

6.99. The K2O/Na2O ratio is 2.42.

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4.1.2 Garnet-biotite gneiss

One sample of garnet bearing gneiss has been analysed and presented in the

Table.4.1.

Gneiss show SiO2 content of 66.98 wt%; Al2O3 content of 9.65wt%; Fe2O3

content of 11.35wt%; the CaO content 1.49wt%; MgO content varies from 5.33wt%;

Na2O contains 1.09wt% and K2O containing is 2.64wt%.

4.1.3 Basic granulites

Basic granulites are mainly garnetiferrous gabbros from Sargur Schist Belts

region. The SiO2 content in basic granulites varying from 50.54 to 50.96wt%; Al2O3

content ranges from 11.16 to 12.19wt%; Fe2O3 content low to higher from 11.50 to

13.20wt%; the CaO content ranges from 11.23 to 14.18wt%; MgO content varies

from 7.01 to 9.34wt%; Na2O contains 2.15 to 2.61wt% and K2O containing is 0 wt%.

The analyses are given in Table.4.1.

In major element discriminate diagrams like SiO2 versus alkalis (Fig. 4.8 and

4.9) basic granulites fall in the field of tholeiitic geochemistry. These basic rocks

exhibits high iron rich tholeiitic geochemistry and fall away from the basaltic

komatiite (BK) or peridotitic komatiite (PK) geochemistry (Fig.4.10, 4.11 & 4.12).

The basic granulite show sub-alkaline nature (Fig.4.16). Pearce (1975) and Mullen

(1983) have utilized some of the major and trace element geochemistry to identify

tectonic setting for basic rocks. These rocks fall away from the Mid Oceanic Ridge

Basalts (MORB) with the exception of only one sample falling in the MORB field

(Fig.4.15). In Harker;s variation diagrams SiO2 versus TiO2, FeO and Al2O3 shows

negative correlation. But SiO2 versus Na2O indicates positive correlation with no

correlation of SiO2 versus Al2O3, CaO, Al2O3, MgO, Na2O and K2O. (Fig.4.17).

4.2 GRANULITES (BRG/MMG)

4.2.1 Charnockites

Representative analyses of charnockites are given in Table 4.2. These include

non-garnetiferrous. The non-garnetiferrous variety dominates the BRG area. Garnet is

normally absent in charnockites in the study area, but are frequent when they are

interbanded with metapelites.

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Geochemistry of three charnockite samples has been analysed and given in the

Table 4.2.

The major element data exhibit a range in SiO2 content from 71.23 to

73.07wt%, Al2O3 from 14.36 to 15.42wt%, MgO from 0.62 to 1.09wt%, Fe2O3 from

1.78 to 2.99wt%, CaO from 2.57 to 3.28wt%. The K2O contents vary from 1.24 to

1.38wt%. Na2O varies from 5.05 to 5.54wt%.

A plot of P2O5/TiO2 versus MgO/CaO (Werner, 1987) and K2O/Al2O3 versus

Na2O/Al2O3 (Garrels and Mackenzie, 1971) diagram the charnockites fall in igneous

field (Fig.4.6 and 4.7) and they show calc-alkaline trend (Fig. 4.1 & 4.4). They are

low potassic and high magnesian in SiO2 versus K2O and FeO versus FeO/MgO

discrimination diagrams (Fig.4.2 & 4.3). The charnockites from the BRG define a

trondhjemitic trend in K-Na-a diagram (Fig.4.5).

4.2.2 Metapelites

Metapelites samples analysed include garnet-cordierite-sillimanite bearing

assemblages. Only one sample from M.M.Hills area has been analysed and presented

in Table 4.2.

The metapelites contains SiO2 of 55.73wt% while Al2O3 21.13wt%. The high

Fe2O3 value of 11.11wt% is reflected by the abundant garnet, cordierite and spinel in

the metapelites. The SiO2/Al2O3 and K2O/Na2O ratios are considered as indicators of

evolutionary changes in the compositions continental crust (Viezer, 1973; Schwab,

1978) and provenance (Condie and Wronkewicz, 1990). SiO2/Al2O3 ratio of this

metapelites is 2.63. The K2O/Na2O ratio is 1.10.

4.2.3 Basic granulites

Chemical analyses of three samples of basic granulites are presented in Table

4.2. These basic granulites are mainly garnetiferrous gabbros in MMG and BRG

region. The SiO2 content in basic granulites varying from 49.61 to 59.85wt%; Al2O3

content ranges from 11.57 to 12.64wt%; Fe2O3 content low to higher from 8.99 to

19.61wt%; the CaO content ranges from9.01 to 12.19 wt%; MgO content varies

from3.38 to 4.49 wt%; Na2O contains 0.52 to 2.85 wt% and K2O containing 0.00 to

0.29wt%.

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In major element discrimination diagrams basic granulites exhibit tholeiitic

geochemistry (Fig.4.9, 4.10 and 4.11)

The basic granulites exhibit high Fe-rich tholeitic geochemistry (Fig.4.10) and

fall away from the basaltic komatiite (BK) or peridotitic komatiite (PK) field

(fig.4.15). These basic rocks show low-K and high magnesian (Fig 4.13 & 4.14) with

sub-alkaline and geochemistry (Fig.4.16). The Harker’s variation diagrams plotted

SiO2 versus TiO2 and FeO indicates negative correlation. But SiO2 versus Al2O3,

CaO, Al2O3, MgO, Na2O and K2O shows no correlation (Fig.4.18).

The present geochemical data for rocks from SSB and BRG/MMG show that

the basic granulites from both the terrane exhibit tholeiitic to Fe-rich tholeiitic trend

and sub alkaline nature in different discrimination diagrams. They fall in the calc-

alkaline basalt field and are magnesian nature with low potassium. The charnockitic

granulites (BRG/MMG) show igneous protolith characteristic and are trondhjemite in

composition.

They do not show any komatiite geochemistry suggesting that they are mainly

plutonic igneous bodies.

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Table. 4.1 Major element analyses for Sargur Schist Belt (SSB)

Rock type Gt-bt gneiss

Metapelite

Basic granulites

Samples CSG 2

CSG 9-1

CSG 42 CSG 45 CSG 66-1 CSG 5

SiO2 66.98

64.53

47.83 50.54 50.96 52.28

TiO2 1.15

1.05

1.24 1.11 0.91 0.69

Al2O3 9.65

16.19

15.42 12.19 11.16 11.29

MnO 0.12

0.09

0.23 0.22 0.20 0.18

Fe2O3 11.35

8.11

15.02 13.2 13.13 11.5

CaO 1.49

0.72

12.98 13.24 11.23 14.18

MgO 5.33

6.11

5.45 7.01 9.34 7.23

Na2O 1.09

0.56

1.31 2.15 2.61 2.56

K2O 2.64

1.91

0.00 0.00 0.00 0.00

P2O5 0.07 0.01 0.10 0.00 0.03 0.00

TOTAL 99.86

99.27

99.57 99.65 99.57 99.9

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Table. 4.2 Major element analyses for Granulites BRG/MMG

Rock type Charnockites

Metapelite

Basic granulites

Samples CSG 69-2 CSG 72

CSG 65

CSG 11-3 CSG 58 CSG 61

SiO2 71.23 73.07

55.73

49.61 59.85 51.62

TiO2 0.31 0.21

1.13

2.11 0.31 1.42

Al2O3 14.36 14.51

21.13

11.57 11.95 12.64

MnO 0.05 0.02

0.09

0.28 0.20 0.36

Fe2O3 2.99 1.78

11.11

19.61 8.99 17.88

CaO 3.28 2.57

1.67

11.03 12.19 9.01

MgO 1.09 0.62

6.39

4.49 4.45 3.38

Na2O 5.05 5.54

1.09

0.52 1.73 2.85

K2O 1.24 1.38

1.20

0.00 0.00 0.29

P2O5 0.06 0.03

0.00

0.21 0.00 0.34

TOTAL 99.65 99.73

99.54

99.42 99.67 99.78

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Fig: 4.1. Plot of SiO2 (wt%) versus (FeO/MgO) wt% (Miyashiro, 1974)

showing calc-alkaline trend for charnockite, BRG/MMG

Fig: 4.2. SiO2 (wt%) versus K2O (wt%) variation diagram (after Rickwood,

1989) for charnockites, BRG/MMG

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Fig.4.3. SiO2 (wt%) versus FeO/(FeO+MgO) wt% diagram for charnockite

BRG/MMG.

Fig.4.4 SiO2 (wt%) versus (Na2O + K2O) wt% to show calcic trend for

charnockite, BRG/MMG

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Fig.4.5. Ab-An-Or plot (after Barker and Oconnor 1965) showing

trondhjemitic trend for charnockite, BRG/MMG

.

Fig.4.6. MgO/CaO versus P2O5/TiO2 diagram to distinguish the ortho and para

nature of charnockitic granulites (Werner, 1987), BRG/MMG.

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Fig.4.7. K2O/Al2O3 versus Na2O/Al2O3 diagram (Garrels and Mackenzie, 1971) to

distinguish the ortho and para nature of charnockitic granulites, BRG/MMG.

Fig: 4.8. FeO-Na2O+K2O-MgO (wt%) plot (Irvine & Baragar, 1971) for basic

granulites showing a distinct tholeiitic trend,( SSB ;BRG/MMG ).

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Fig: 4.9. SiO2 versus (FeO/MgO) wt% diagram for basic granulites showing

a tholeiitic trend, (SSB ;BRG/MMG ) (after Miyashiro, 1974).

Fig.4.10. Al2O3 – (FeO+ TiO2)- MgO ternary diagram (Jensen,1976) showing the

different types of basalts and tholeiitic fields for basic granulites.

SSB ( ) and BRG/MMG ( )

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Fig.4.11. Cation diagram after Jensen (1976), modified by Viljoen et al. (1982) showing

Fe-tholeiite trend for basic granulites, SSB ( ) and BRG/MMG ( ).

Fig.4.12. CaO–MgO–Al2O3 plots after Viljoen & Viljoen (1969) and Viljoen et al.,

(1982) for basic granulites of SSB ( ) and BRG/MMG ( ).

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Fig: 4.13. SiO2 versus K2O wt% variation diagram (after Rickwood, 1989)

for basic granulites, (SSB- ; BRG/MMG- ).

Fig: 4.14. SiO2 (wt %) versus FeO/(FeO+MgO) diagram showing magnesian

nature of basic granulites, (SSB- ;BRG/MMG- ).

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Fig.4.15. MnO*10 – TiO2 – P2O5*10 ternary diagram (Mullen, 1983) showing the

different tectonic types of basalts and tholeiitic fields for basic granulites,

(SSB- ; BRG/MMG- ).

Fig.4.16. Plot of SiO2 versus Na2O + K2O (wt %) to know the alkaline and

subalkaline nature of basic granulites, (SSB- ; BRG/MMG- ).

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Fig.4.17. Harker’s variation diagrams for basic granulites, SSB.

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Fig.4.17. Harker’s variation diagrams for basic granulites, SSB.

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Fig.4.18. Harker’s variation diagrams for basic granulites, BRG/MMG.

150

Fig.4.18. Harker’s variation diagrams for basic granulites, BRG/MMG.